Peter Frost's anthropology blog, with special reference to sexual selection and the evolution of skin, hair, and eye pigmentation

Tuesday, October 28, 2008

Skin color and vitamin D

Differences in human skin color are commonly explained as an adaptive response to solar UV radiation and latitude. The further away from the equator you are, the weaker will be solar UV and the less your skin will need melanin to prevent sunburn and skin cancer.

A variant of this explanation involves vitamin D, which the body needs to make strong bones and which the skin produces with the help of UV-B. The further away from the equator you are, the lighter your skin will be to let enough UV-B into its tissues for vitamin D production. Or so the explanation goes.

To test this hypothesis, Osborne et al. (2008) measured skin color and bone strength in a hundred white and Asian adolescent girls from Hawaii. Skin color was measured at the forehead and the inner arm. Bone strength was measured by section modulus (Z) and bone mineral content (BMC) at the proximal femur. A multiple regression was then performed to investigate the influences of skin color, physical activity, age, ethnicity, developmental age, calcium intake, and lean body mass on Z and BMC. Result: no significant relationship between skin color and bone strength.

Is there, in fact, any hard evidence that humans vary in skin color because they need to maintain the same level of vitamin D production in the face of varying levels of UV-B? Robins (1991, pp. 204-205) found the data to be unconvincing when he reviewed the literature. In particular, there seems to be little relationship between skin color and blood levels of 25-OHD—one of the main circulating metabolites of vitamin D:

The vulnerability of British Asians to rickets and osteomalacia has been ascribed in part to their darker skin colour, but this idea is not upheld by observations that British residents of West Indian (Afro-Caribbean) origin, who have deeper skin pigmentation than the Asians, very rarely manifest clinical rickets … Moreover, artificial irradiation of Asian, Caucasoid and Negroid subjects with UV-B produced similar increases in blood 25-OHD levels irrespective of skin pigmentation … A study under natural conditions in Birmingham, England, revealed comparable increases in 25-OHD levels after the summer sunshine from March to October in groups of Asians, West Indians and Caucasoids … This absence of a blunted 25-OHD response to sunlight in the dark-skinned West Indians at high northerly latitudes (England lies farther north than the entire United States of America except for Alaska) proves that skin colour is not a major contributor to vitamin D deficiency in northern climes.

The higher incidence of rickets in British Asians probably has less to do with their dark color than with their systematic avoidance of sunlight (to remain as light-skinned as possible).

Skin color and natural selection via solar UV

Solar UV seems to be a weak agent of natural selection, be it through sunburn, skin cancer, or vitamin D deficiency. Brace et al. (1999) studied skin color variation in Amerindians, who have inhabited their continents for 12,000-15,000 years, and in Australian Aborigines, who have inhabited theirs for some 50,000 years. Assuming that latitudinal skin-color variation in both groups tracks natural selection by solar UV, their calculations show that this selection would have taken over 100,000 years to create the skin-color difference between black Africans and northern Chinese and ~ 200,000 years to create the one between black Africans and northern Europeans (Brace et al., 1999). Yet modern humans began to spread out of Africa only about 50,000 years ago. Clearly, something other than solar UV has also influenced human variation in skin color ... and one may wonder whether lack of solar UV has played any role, via natural selection, in the extreme whitening of some human populations.

Indeed, people seem to do just fine with a light brown color from the Arctic Circle to the equator. Skeletal remains from pre-contact Amerindian sites show little evidence of rickets or other signs of vitamin D deficiency—even at latitudes where Amerindian skin is much darker than European skin (Robins, 1991, p. 206).

Why, then, are Europeans so fair-skinned when ground-level UV radiation is equally weak across Europe, northern Asia, and North America at all latitudes above 47º N? (Jablonski & Chaplin, 2000). Proponents of the vitamin D hypothesis will point to the Inuit and say that non-Europeans get enough vitamin D at high northerly latitudes from fatty fish. So they don’t need light skin. In actual fact, if we look at the indigenous peoples of northern Asia and North America above 47º N, most of them live far inland and get little vitamin D from their diet. For instance, although the Athapaskans of Canada and Alaska live as far north as the Inuit and are even somewhat darker-skinned, their diet consists largely of meat from land animals (caribou, deer, ptarmigan, etc.). The same may be said for the native peoples of Siberia.

Conversely, fish consumption is high among the coastal peoples of northwestern Europe. Skeletal remains of Danes living 6,000-7,000 years ago have the same carbon isotope profile as those of Greenland Inuit, whose diet is 70-95% of marine origin (Tauber, 1981). So why are Danes so light-skinned despite a diet that has long included fatty fish?

Skin color and sexual selection via male choice

Latitudinal variation in human skin color is largely an artefact of very dark skin in sub-Saharan agricultural peoples and very light skin in northern and eastern Europeans. Elsewhere, the correlation with latitude is much weaker. Indeed, human skin color seems to be more highly correlated with the incidence of polygyny than with latitude (Manning et al., 2004).

This second correlation is especially evident in sub-Saharan Africa, where high-polygyny agriculturalists are visibly darker than low-polygyny hunter-gatherers (i.e., Khoisans, pygmies) although both are equally indigenous. Year-round agriculture allows women to become primary food producers, thereby freeing men to take more wives. Thus, fewer women remain unmated and men are less able to translate their mate-choice criteria into actual mate choice. Such criteria include a preference, widely attested in the African ethnographic literature, for so-called 'red' or 'yellow' women — this being part of a general cross-cultural preference for lighter-skinned women (van den Berghe & Frost, 1986). Less mate choice means weaker sexual selection for light skin in women and, hence, less counterbalancing of natural selection for dark skin in either sex to protect against sunburn and skin cancer. Result: a net increase in selection for dark skin.

Just as weaker sexual selection may explain the unusually dark skin of sub-Saharan agricultural peoples, stronger sexual selection may explain the unusually light skin of northern and eastern Europeans, as well as other highly visible color traits.

Among early modern humans, sexual selection of women varied in intensity along a north-south axis. First, the incidence of polygyny decreased with distance from the equator. The longer the winter, the more it cost a man to provision a second wife and her children, since women could not gather food in winter. Second, the male death rate increased with distance from the equator. Because the land could not support as many game animals per unit of land area, hunting distance increased proportionately and hunters more often encountered mishaps (drowning, falls, cold exposure, etc.) or ran out of food, especially if other food sources were scarce.

Sexual selection of women was strongest where the ratio of unmated women to unmated men was highest. This would have been in the ‘continental Arctic’, a steppe-tundra environment where women depended the most on men for food and where hunting distances were the longest (i.e., long-distance hunting of highly mobile herds with no alternate food sources). Today, this environment is confined to the northern fringes of Eurasia and North America. As late as 10,000 years ago, it reached much further south. This was particularly so in Europe, where the Scandinavian icecap had pushed the continental Arctic down to the plains of northern and eastern Europe (Frost, 2006).

The same area now corresponds to a zone where skin is almost at the physiological limit of depigmentation and where hair and eye color have diversified into a broad palette of vivid hues. This ‘European exception’ constitutes a major deviation from geographic variation in hair, eye, and skin color (Cavalli-Sforza et al., 1994, pp. 266-267).

This is interesting information that lends new credibility to the vitamin D hypothesis of European skin lightening. Cereals contain phytic acid, which binds to calcium and impedes calcium absorption. This calcium deficit may, in turn, increase selection for more vitamin D synthesis and, hence, lighter skin.

I will read over the references this weekend. Offhand, the main problem I have with this 'revised' hypothesis is that European skin would have whitened very recently, i.e., with the coming of cereal agriculture. In northern Europe, this would be about five thousand years ago.

Research is now ongoing to pin down the time frame when European skin turned white. Hopefully, we will soon have an answer.

About the sexual selection hypothesis: why is it not taught in school to explain ethnic differences ?

We know why. Anyway, I understand the polygyny point, but it doesn't go far enough.Why would a hunter select to provide to fair skin mate ?When you mate, it's not for romance, it's to have kids. You don't really select a mate. You select the genes that you want, or you need, in your kids.But white or dark, a kid is still at the charge of a provider, and if the fair kid is no better at hunting, what good does it make for the father ?

So, basically, human bred themself like they did for their dogs, with no other purpose than to be as fair as possible, correct ?

There is something missing. Here is my conjecture. Hominids had always favorised neotenic traits because in doing so, they favorise youthness, that is, Life. Maybe just because being aware of it, honinids fear death. I do.And an old hominid is closer to death than a young one. He looks like death, he smells like death.As a side effect, this tendancy to choose Life could have powered the encephalisation increase, along with the life expectency, in all the hominid genus way before sapiens appeared. And as another side effects, fair skin, hairs and eyes could have been selected as part of these neotenic traits. They are just part of the fontain of eternal youth.

But you can only select for the characters that you have, not for the characters that you wish to have, to quote Rumsfeld. For exemple, according to the neotenic theory, asians have the most neontenic traits, so they must have practiced a strong sexual selection. But they never get the european fair skin because this trait never appeared in their genes.

So, as always in evolution, there is contingencies AND luck.By the way, did you know that Darwin said that struggle for life is not just a matter of luck of being the fittest. It is also a matter of being happy ????In its Origin of Species, he said, quote: "When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healty, and the happy, survive and multiply."

Food and Evolution by Marvin Harris p 81"Among Europeans and possibly other populations, the abilty of adults to produce lactase, the enzyme necessery for digesting lactose, appears to have evolved during the spread of an early neolithic dairying complex. Because of the clothing they wore and their cloudy days, long nights, and long winters, early neolithic peoples of Europe were prone to vitamin D deficiency. Vitamin D is essential for the absorbion of calcium and the prevention of rickets and osteomalacia. Lactase promotes the absorbtion of calcium and hence can play a role in compensating for vitamin D deficient diets, resulting in the selection of the gene for lactase sufficiency in adulthood (Simoons 1982:215)*. This scenario explains why Europeans have a preference for and Chinese an aversion to, fresh liquid milk".

According to Nick Lane in Power, Sex, and Suicide, the mitochondria of north Europeans has been found to produce more heat by uncoupling than those of southern Europeans. He says this seems to be a cold adaptation, one that sacrifices power for heat generation and burns more calories. He speculates that this may be one reason why the Inuit suffer relatively little type 2 diabetes ect.

The heat producing cold adapted mitochondia of northern Europeans are less powerful, due to energy dissapated as heat which usually doesn't matter, however in sperm, which only have a about 150 mitochondria each, a type of male infertility can result from underpowered sperm. The opposite should be true of those mitochondria that evolved in hot climates, and Africans Americans do seem to suffer from t type 2 diabetes ect. more than other peoples.

Going by the theory expounded in the evolution of polyandy paper above, those sperm (and the mitochondia powering them) being less well adapted to compete should have been eliminated from the gene pool. That they have escaped this fate in northern Europe is not without significance.

Many authors going back to Darwin have tried to use sexual selection to explain differences in physical appearance among human populations. Most of them, however, simply argued in terms of differences in sexual preferences. In general, the literature suggests that geographic variation in sexual preferences is relatively minor and cannot explain the much larger variation in human physical appearance.

My own work has been on a modified version of the 'sexual selection hypothesis', sexual selection has shaped geographic variation in physical appearance through differences in the intensity and direction of sexual selection. I discussed this hypothesis in my 2006 paper in Evolution and Human Behavior. Another paper on the same subject will come out shortly.

Most of what you say is true for relatively low intensities of sexual selection. At low intensities, a mate is chosen on the basis of physical characteristics that indicate mate quality. At high intensities, i.e., when one sex is in a buyer's market, selection is more strongly influenced by physical characteristics that attract attention in one way or another, often by hyperstimulating a mental algorithm that is not directly related to mate choice.

Tod,

The problem I have with Harris is that white skin does not seem to be better at vitamin D production than brown skin. I am aware of the evidence that melanin blocks UV light and that UV light is needed for vitamin D synthesis, but the difference in UV transmission between white and brown skin doesn't seem to matter a lot. See my next post.

The claim is that haplogroup T (and/or sublineages of haplogroup U) are overrepresented in asthenozoopermic men. Those studies have beendebunked.

The opposite should be true of those mitochondria that evolved in hot climates, and Africans Americans

"In our study, the samples from the infertile males with less than 50% progressive spermatozoa (the asthenozoospermic ones) belonged to diverse lineages of several haplogroups: L2a1, I1a, X2b, both T1 and T2, J1c, U5a, K1 (K1*, K1a4, K1b1a, and K1c1), V1a, H*, and H1. Note that the first example of a sub-Saharan L2a lineage in asthenozoospermic males can hardly be reconciled with the driving selective force of cold adaptation advanced in (Montiel-Soza et al. 2006)."

Going by the theory expounded in the evolution of polyandy paper above, those sperm (and the mitochondia powering them) being less well adapted to compete should have been eliminated from the gene pool. That they have escaped this fate in northern Europe is not without significance.

Well OK decoupled, heat generating mtDNA isn't very obvious now many generations after the Ige Age it was presumably adapted to. However, that doesn't mean it never existed, mtDNA that formerly led to the lessened fertility may have ceased to exist in warmer conditions in competition with other types more suited to to the conditions. When cold adaptation became maladaptive because it reduced fertility the most cold adapted mtDNA may have died out

This article regarding black people having more wives causing blacker skin is a farce of nonsense.

Greenland is a cold climate and therefore the Hunters in Greenland should have picked white blue eyed mates, or developed white blue eyed people over time. This is not the case.

The black people having more wifes explains absolutely nothing and is a bunch of garbage pseudoscience.

Note that I am a white blue eyed, light hair color person myself. I find this nonsense to be absolutely false and ridiculous.

So the people in Greenland cheated on their husbands and had multiple wives more often than Icelanders, and this cheating caused brown skin in greenland and white skin in Iceland? Cheating, the process of having multiple wives, causes black skin? what a bunch of utter horse baloney.

Let us look at some Greenland family marriage statistics and verify that Inuit and Greenland people tend to have multiple wives and cheat on their partners. And some how this cheating, putting ones dick into multiple pink pleasure holes, causes black skin. True horse baloney and brown malarkey!

White snow, white skin, camouflage for hunting or raiding villages by surprise attacks. Still does not explain the greenlanders, who have dark skin.. however the greenlanders did not do surprise attacks and maybe they fished more than hunted. However icelanders probably fish more than they hunt and icelanders are very blonde/blue eyed, the most in the world in fact.

Completely confusing rubbish, skin color and eyecolor. Absolute confusing malarky. Do not take this as an attack - it is just so confusing that I must admit it outright.

In all likelihood human evolution is simply based on gradual hair loss that exposed more skin, more exposed skin produced more vitamin D, more vitamin D produced more serotonin, and serotonin drove brain development and behavior.

Since hair loss follows maternal genetic line, we could even give credit to a female chimanzee like animal with a genetic fault regarding hair which has been ampflied over the millenia.

"A team of scientists has tracked down a genetic mutation that leads to blue eyes. The mutation occurred between 6,000 and 10,000 years ago, so before then, there were no blue eyes... If the OCA2 gene had been completely shut down, our hair, eyes and SKIN would be melanin-less, a condition known as ALBINISM."http://www.msnbc.msn.com/id/22934464/wid/11915773

SLC45A2: "Mutations in this gene are a cause of oculocutaneous ALBINISM type 4."http://en.wikipedia.org/wiki/SLC45A2

The SLC24A5 gene is located on the long (q) arm of chromosome 15 on position 21.1, from base pair 46,200,461 to base pair 46,221,881.

It is currently estimated that the threonine allele became predominant amongst Europeans 5,300 to 12,000 years ago.http://en.wikipedia.org/wiki/SLC24A5

Chromosome 15 has several other disorders that have characteristic light skin and hair.

"The OCA2 gene is located in a region of chromosome 15 that is often deleted in individuals with Angelman syndrome. A loss of this gene does not cause the characteristic features of Angelman syndrome; however, people who are missing one copy of this gene tend to have unusually light-colored hair and fair skin"http://ghr.nlm.nih.gov/gene=oca2

The region of chromosome 15 containing the OCA2 gene is often deleted in individuals with Prader-Willi syndrome. A loss of this gene does not cause the characteristic features of Prader-Willi syndrome; however, people who are missing one copy of this gene tend to have unusually light-colored hair and fair skinhttp://ghr.nlm.nih.gov/gene=oca2

"More than 70 mutations in the OCA2 gene have been identified in people with oculocutaneous albinism type 2. People with this form of albinism often have light yellow, blond, or light brown hair, creamy white skin, light-colored eyes, and problems with vision. The most common OCA2 mutation is a large deletion in the gene, which is found in many affected individuals of sub-Saharan African heritage. Other OCA2 mutations, including changes in single DNA building blocks (base pairs) and small deletions, are more common in other populations.http://ghr.nlm.nih.gov/gene=oca2

Notice how in the last passage they specifically state "which is found in many affected individuals of sub-Saharan African heritage." but fail to name the other populations - Europeans who have "changes in single DNA building blocks (base pairs) and small deletions"

Yet below says it all:

Research done by a large team at Penn State University and a number of other institutions discovered that the gene has two primary alleles that differ in only one nucleotide, changing the 111th amino acid from alanine to threonine. [1][2][3]

The threonine allele was present in 98.7 to 100% among several European samples, while the alanine form was found in 93 to 100% of samples of Africans, East Asians and Indigenous Americans.http://en.wikipedia.org/wiki/SLC24A5

Hmm.. I debated with myself as to whether or not I would actually speak up but, as I am not one to stat silent, at least not for long haha I feel as though I should do so. From living the life I have lived to date and much research into several different aspects of history, science, genetics, as well as many other things I have come to a conclusion that a majority of our more humanistic characteristics have been shaped by environment, foods, the soil in which we eat, breath, and live on, breeding habits, along with something I haven't seen to much discussion on which is viruses, bacteria, and diseases that have more than just a minor impact on past present and future mutations of our species. Viruses in the human species can be tracked far through our genes even though they have not been given much credit they have shaped humans and animals alike in sometimes similar or opposite ways but, have had impact to various degrees. I was just doing some research into albinism due to the fact that I was born VERY light skinned and had almost white hair and blue almost clear eyes as a child. I was often referred to as a toe head as I wasn't quite completely albino. The odd part was I became very ill, running a temp upwards of 104-106 for a bit which shocked everyone at the time due to the fact that it should have caused alot of damage to my brain and most likely death etc. Not long afterwards my hair, eye, and skin color began to change. Though one odd effect still remains even though to date it is known to be only due to the melanin etc in the eyes that causes change in eye color, my eyes do in fact change color under controlled lighting and environment between solid gold, dark green like jade, as well as to clear blue... Though it is supposed to be impossible, it is not only quite possible but, is a reality for me. These days I prefer to wear sunglasses as I tired of people starring as such things would happen as my emotions changed so does that. I refused to become apart of the case study that was requested long ago even though I underwent many tests and to they're amazement at the time I do not exist you might say but, if I pinch myself I do actually. I also refused to sign any waivers that would allow my records or information to be shared or discussed or to be poked and prodded again.... The truth of life is very simple..and the answers are the same.. Life exists no matter where you go and what you do to it, it will survive in one shape or another however it has to..

Welcome to my blog! For the most part, this page will be an extension of my website, with comments relating to my research. But it will also branch out into more general discussions of human evolution.